1. Field of the Invention
The present invention relates to printing systems capable of bidirectional printing. Specifically, the present invention concerns a system in which either bidirectional or unidirectional printing is selectively performed based on whether data to be printed includes data other than black print data.
2. Description of the Related Art
A conventional serial printing system prints images on a recording medium by scanning a printing unit across the medium. During scanning, the printing unit places ink on the medium according to received print data. After completion of one scan, the medium is advanced with respect to the printing unit. This process continues until an entire page of print data is printed.
In unidirectional serial printing, the printing unit, while printing, scans across the recording medium from a home end of the medium to a far end of the medium. After reaching the far end of the medium, the unit moves, without printing, back to the home end. Bidirectional printing differs from the foregoing in that printing occurs also during movement of the printing unit from the far end of the medium to the home end. Accordingly, it takes more time to print a particular page of data using unidirectional printing than when using bidirectional printing.
Although bidirectional printing provides greater throughput than unidirectional printing, bidirectional printing has some drawbacks. Most notable is that data printed during a backward scan is often of different quality than data printed during a forward scan. Several factors may contribute to this difference. For example, a leading edge of the printing unit during the forward scan becomes the trailing edge during the backward scan, therefore printing elements of the printing unit must be extremely symmetrical to avoid creating differences between forward-printed data and backward-printed data. As another example, the printing unit may be disposed at a distance from the recording medium during the forward scan which is different from the corresponding distance during the backward scan, due to vibrations or other mechanical factors.
In addition, in the case of ink jet printing systems, a droplet of ink ejected to a desired location is often accompanied by smaller "satellite" droplets which, during a forward scan, often merge with the droplet at the desired location but which, during a backward scan, extend past the desired location in the direction of the backward scan. It should be noted that differences in forward-printed and backward-printed portions of an image due to such satellites are most noticeable in the case of inks having a high optical density.
In view of the foregoing problems, what is needed is a system for quickly printing data onto a recording medium while minimizing image gradations caused by bidirectional printing.